Sayyiduna Ibn Abbas (may Allah be pleased with them both) says: "Declaring (something) unlawful is an oath."
And Sayyiduna Ibn Abbas (may Allah be pleased with them both) said (as evidence for it): "Indeed, in the Messenger of Allah (peace and blessings be upon him) you have an excellent example."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12408